In recent years, meta-material techniques have been proposed in which the dispersion relation of electromagnetic waves which propagate using a structural body as a medium is artificially controlled, and technological applications have been examined in a variety of fields. The dispersion relation means a relationship between the wave number (or wavelength) and the frequency of propagating electromagnetic waves. In structural bodies in which a conductor pattern or a conductor structure is periodically arranged, it is possible to control the dispersion relation by appropriately setting the dimensions or the placement, physical properties and the like of various types of elements.
As one meta-material, a transmission line which is a composite right and left handed (simply referred to as CRLH) medium is known. In the CRLH transmission line, it is known that incident electromagnetic waves indicate properties as a right-handed medium, a left-handed medium, and an electromagnetic bandgap structure (hereinafter, simply referred to as an EBG structure) depending on the frequency. The general CRLH transmission line is configured such that a unit structure is periodically disposed one-dimensionally or two-dimensionally. A mushroom-type structure which is one type of unit structure has a structure in which a conductor patch on the surface of a dielectric layer and a conductor plane on the backside of the dielectric layer are electrically connected to each other by a conductor via and the like passing through the dielectric layer.
Incidentally, a structure having a dispersion relation in which the propagation of electromagnetic waves in a specific frequency band is suppressed is called an EBG structure. When the EBG structure is installed on the board such as a printed board or a device package board, it is possible to suppress the propagation of electromagnetic waves generated in the surface of the board, and to reduce, for example, the electromagnetic interference between the antennas or between the devices mounted on the board. It is known that the EBG structure functions as a magnetic wall that reflects incident electromagnetic waves in phase in the vicinity of the bandgap. When the EBG structure is installed on the back side of the antenna using such characteristics, it is possible to realize the low profile of the antenna without lowering the radiation efficiency.
In the CRLH transmission line, capacitance components formed between the adjacent conductor patches constitute an LC parallel resonance circuit along with inductance components formed from the conductor via. The CRLH transmission line has a bandgap in the vicinity of the resonance frequency, and thus can be used as the EBG structure. It is preferable to increase the inductance components in order to shift this bandgap to a lower frequency. However, when the conductor via is made longer in order to increase the inductance components, the thickness of the structure increases.
Techniques capable of solving such a disadvantage include, for example, techniques disclosed in Patent Documents 1 and 2. In Patent Document 1, an intermediate layer in which a planar inductance element such as a spiral inductor is disposed is provided between a conductor patch layer and a conductor plane layer. The inductance element is connected to a patch layer or a conductor plane through the conductor via. The planar inductance element is provided, whereby it is possible to increase the inductance components without increasing the thickness of the EBG structure.
An antenna disclosed in Patent Document 2 is configured to use line-length resonance in the frequency band in which the CRLH transmission line operates as a left-handed medium. In a normal medium (right-handed medium), as the frequency becomes lower, the wavelength of electromagnetic waves becomes longer. Therefore, the size of the antenna structure is increased. In a left-handed medium, as the frequency becomes lower, the wavelength of electromagnetic waves becomes shorter. Therefore, it is possible to miniaturize the antenna by using the CRLH transmission line as a left-handed medium. In Patent Document 2, a slit is provided in the vicinity of the connection portion of the conductor plane and the conductor via and a coplanar line is formed. The coplanar line is provided, whereby it is possible to increase the inductance components without increasing the thickness of the antenna.